With development of industry and population increase, power consumption has rapidly increased but power generation has some limitations.
Therefore, a power system for stably supplying power generated in a generation area to a consumption area without loss has gradually become important.
A necessity of a flexible AC transmission system (FACTS) for improving power flow, system voltage and stability has arisen. A STATic synchronous COMpensator (STATCOM) which is a third-generation power compensator of a FACTS is connected to a power system in parallel to compensate for reactive power and active power necessary for the power system.
FIG. 1 shows a general power supply system.
As shown in FIG. 1, the general power supply system 10 may include a power source 20, a power system 30, a load 40 and a plurality of reactive power compensators 50.
The power source 20 refers to a place or apparatus, in which power is generated, and may be understood as a generator for generating power.
The power system may mean all apparatuses including a power line, a steel tower, an arrester and an insulator, for transmitting power generated by the power source 20 to the load 40.
The load 40 means a place or apparatus for consuming power generated by the power source 20 and may be understood as a consumer for consuming power.
The reactive power compensator 50 is a STATCOM, which is connected to the power system 30 to compensate for lack of reactive power upon lack of reactive power in power flowing to the power system 30.
The reactive power compensator 50 includes a converter for converting AC power of the power system into DC power or converting DC power into AC power.
The converter includes clusters each including a plurality of cells connected to each other in series with respect to three phases.
FIG. 2A is a circuit diagram of a converter having a star connection topology and FIG. 2B is a circuit diagram of a converter having a delta connection topology.
As shown in FIGS. 2a and 2b, the plurality of cells 54 is respectively connected to each other in each of three-phase clusters 53.
In a converter having a star connection topology or a converter having a delta connection topology, in order to obtain high reliability and excellent driving performance, uniform energy control among clusters and uniform voltage control among the cells of the cluster are important.
For uniform energy control among the clusters, a zero-sequence voltage is used in the converter having start connection topology and zero-sequence current is used in the converter having the delta connection topology.
In the related art, in a uniform control method using a zero-sequence voltage, control is impossible and calculation is complicated when current is not supplied.
In the related art, in a uniform control method using zero-sequence current, a negative sequence component is not considered and thus reliability of uniform control is lowered.
In the related art, in the uniform control method, since zero-sequence voltage and current are calculated using error values among energies of clusters, when external disturbance occurs, a large overshoot or transient voltage may be generated. Accordingly, if a withstand voltage of a capacitor or a switching element of a cell (submodule) is low, the system cannot continuously operate.